Effect of irradiation on the microstructure of Nicalon fibers

Microstructural analyses were performed by TEM on two types of Nicalon fibers (CG and Hi) after neutron irradiation at nominally 1040 degrees C to a relatively high dose (43 dpa). For comparison, microstructural analyses also were performed on unirradiated fibers that were thermally annealed at 1010 degrees C for a time equivalent to the irradiation exposure time. No grain growth was observed for either type of unirradiated, but thermally annealed Nicalon fibers. However, significant grain growth was observed to have taken place in the irradiated Nicalon-CG fiber, presumably irradiation induced. In contrast, no significant amount of grain growth was observed in the irradiated Hi-Nicalon fibers. Void and other irradiation defect structures were not observed in either fiber. For such extreme irradiation conditions (43 dpa at 1040 degrees C), the Hi-Nicalon fiber exhibits a much higher degree of microstructural stability than Nicalon-CG fiber. Composite SiC/SiC made with Hi-Nicalon promises to also exhibit improved irradiation performance.